One of the central features of language, like other complex cognitive systems, is productivity – language doesn’t just consist of a stored list of things to say, but rather provides a rich system of patterns or rules that let speakers express potentially infinite meanings with finite means.
For example, you’ve learned that one of these is called a “dog”, and multiple of them are called “dogs”. You can guess that if I tell you that one of these is called a “wug”, multiple of them are called… “wugs”!
